Automatic battery charging system for a battery back-up DC power supply

ABSTRACT

An automatic back-up battery management system utilizing primary DC power for telephone switching equipment, communications equipment, computer equipment or other loads.

This is a continuation of provisional patent application No. 60/219,825,filed Jul. 21, 2000.

FIELD OF THE INVENTION

The present invention relates to a back-up battery management system foruse with a primary DC power supply for telephone switching equipment orother loads, such as communication and computer equipment. In many otherapplications, the need for an uninterrupted source of DC power iscritical.

BACKGROUND OF THE INVENTION

To avoid any interruption or outage in power service, it is commonpractice to employ a battery back-up for the primary DC source. Back-upbattery systems typically include strings of batteries or cellsconnected in parallel with the primary DC source and the load. In theevent of a drop in the load bus voltage below a predetermined threshold,the back-up battery supplants or supplements the primary source of DCpower. Back-up battery systems are designed to replace the primary DCpower source for a predetermined period of time within which resumptionof primary power is expected to occur.

In conventional back-up battery systems, the nominal system load busvoltage has typically been dictated by battery characteristics. Forexample, in a telephone switching plant, back-up batteries are commonlyemployed which have a design cell voltage of 2.25 volts for optimumhealth of the battery cell. Twenty-four cells are typically combined ina string resulting in a nominal load bus voltage of approximately −54volts. A bank of strings supplies the necessary back-up DC power.

As the back-up batteries are placed across the load, the full 54 voltsof system DC voltage are placed across the battery string. This designarchitecture of a typical back-up battery system presents a number ofpotential problems. Certain batteries, due to their electrochemicalconstitution, will draw more current than other batteries. Allbatteries, as they age, will experience increasing internal resistanceand will draw more charging current from the main DC supply.

About a decade ago, a new type of lead acid battery was introduced intothe marketplace. The battery is sealed, and allegedly requires nomaintenance. In this type of battery, oxygen and hydrogen producedduring electrochemical reactions in the battery recombine to maintain anaqueous liquid electrolyte at a constant level within the cell. As aresult, these batteries have only a small amount of liquid electrolyte.These batteries have become known as “valve regulated” or “recombinant”or “electrolyte-starved” batteries.

This type of lead acid battery (hereinafter termed “valve regulated leadacid” or “VRLA” batteries) has often failed well before their designlife, which is typically 10 years.

A particular battery may, for various reasons not clearly understood,begin to take on more amperage to maintain its charge. The increasedcharging current will elevate the temperature of the battery. Thechemical recombination of the oxygen and hydrogen gases also createsheat. As the internal battery temperature increases, the current demandincreases disproportionately. For every 10 degrees centigrade ofincrease in the battery's internal temperature, the battery demand forcurrent doubles. A battery in this condition will have one of twofailure modes, the most damaging being “thermal runaway.” Thermalrunaway may lead to an explosion of the battery, with likely destructionor severe damage to any nearby equipment. Alternatively, the battery mayexperience a “melt down” and produce noxious gases that also are apt todamage or destroy neighboring equipment. The rectified AC sourceprovided in typical telephone switching plants has more than amplecapacity to supply any one or more batteries demanding abnormal chargingcurrent. This, together with paralleled battery strings, encourages thepreviously described thermal runaway or meltdown failures.

With the advent of fiber optic signal distribution, switching equipmenthas been decentralized, introducing a need for DC power supplies inunattended satellite installations distributed throughout the territoryserved. In these unattended installations, the equipment is oftenclosely packed, leading to hostile thermal operating conditions for theequipment and increased occurrences of thermally induced failures. Inless severe conditions, the placement of the back-up batteries directlyacross the load is apt to result in dry-out (loss of electrolyte),positive grid corrosion, and other problems which may lead to prematurebattery failure and/or below normal power performance.

Back-up battery systems must be monitored to determine the health andcapacity of the batteries. The need to perform battery tests isparticularly troublesome in systems that require the supply of anuninterrupted source of DC power. Testing of the vital statistics of abattery affecting output capacity, predicted life, etc. is presentlydone by taking a battery string off-line and testing it in one of twoways. The test procedure recommended by battery manufacturers as beingthe most reliable, is to discharge the battery into a load whilemeasuring the response of the battery. The ability of a battery orbattery string to hold a predetermined current level for a predeterminedtime is a reliable measure of the health and capacity of the battery.However, such discharge tests in the field require experienced personneland are difficult and costly. Further, conventional battery testing,requiring the batteries to be taken off-line, suffers a loss of standbybattery protection for the telephone plant or other equipment beingsupplied while the tested batteries are off-line.

To avoid the cost and inconvenience of a discharge test, it iscommonplace to employ special field test equipment that tests forbattery resistance, impedance, inductance and other parameters andcharacteristics without discharging the battery. See U.S. Pat. No.5,250,904. However, as noted, tests that do not involve discharging thebattery are apt to be less reliable.

SUMMARY OF THE INVENTION

U.S. Pat. No. 5,160,851 discloses a back-up battery system for telephonecentral office switching equipment. The back-up battery system includesthat when the batteries are switched in circuit across the load, thecumulative battery voltage exceeds a predetermined load voltage for aselected period of time. A converter down converts the over voltage thatresults from switching extra cells across the load. The converter, asensor for sensing the system discharge bus voltage and a switch may beformed as a single unit using MOSFET technology. One or morerechargeable batteries have cells floated at a given float voltage. Afail-safe contact switch may also be provided to parallel the MOS-FETswitch and be operated in the event of MOS-FET failure.

Also, U.S. Pat. No. 5,777,454 discloses a back-up battery managementsystem for use in a DC power supply system for use with telephoneswitching equipment or loads of other types. The disclosed batterymanagement system is particularly adapted for use with batteries of thevalve regulated lead acid type. It can also find utility with older“flooded” lead acid type batteries and batteries of other types.

The disclosed invention also provided a back-up battery managementsystem for a battery backed-up primary DC power supply that permittedthe back-up batteries to be maintained on-line at all times. The back-upbattery management system also included means for charging the batterieswith a predetermined level of substantially constant current whileisolating the batteries from the system load bus. The charging currentis substantially constant at a given time and for a given condition ofthe battery.

The invention further disclosed that a control system is provided whichmonitors and controls all significant conditions and parameters withinthe back-up battery management system to maintain the battery system ata float charge during normal operation.

The present invention (ABC System) also concerns a back-up batterymanagement system for use in a DC power supply for use with telephoneswitching equipment and loads of other types. However, the methods usedto isolate the batteries from the load bus and administer differentlevels of charging current into the batteries differ substantially fromthe previously disclosed inventions by the inventor. These differenceswill become apparent when the circuitry is described and the claims areput forth.

The invention and its particular features and advantages will becomemore apparent from the following detailed description considered withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the Automatic Battery Charging(ABC) System.

FIG. 2 is a schematic block diagram of the Automatic Battery Charging(ABC) System.

FIG. 3 is a schematic block diagram of the Automatic Battery Charging(ABC) System.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the preferred embodiment of the (ABC System) inventionsupplying charging power for the battery strings 12 and 13 via a “SpareBus” DC power source. The “Spare Bus” DC power source is comprised of aSecondary DC power source 2 providing power to a Spare bus 55. Includedis voltage adjust circuitry 56, a feedback connection 57 between theSecondary DC power source 2 and the voltage adjust circuitry 56 andfinally by control signals via 38 from a Controller 3.

FIG. 3 shows the preferred embodiment of the ABC System invention, as itwould appear with a third added battery string 14. It is otherwiseessentially the same as FIG. 2.

In FIG. 2, showing the preferred embodiment of the ABC system invention,a primary DC source 8 supplies DC power to a system load 10 through asystem load bus 11. The primary DC source 8 may comprise a conventionalsystem for developing rectified DC power from a commercial source of ACpower. A system for use in a telephone switching office may employ abank of battery strings. In the illustrated embodiment, two exemplarystrings of battery are shown schematically at 12 and 13. In a typicalinstallation, each battery string 12 and 13 comprises a number of cellsor group of 6 cells (termed mono-blocks) 16 herein sometimes referred toas “cells/ mono-blocks.” As shown each cell may produce for example,−2.25Vdc. In a bank of 24 cells on float charge, a total voltage levelapproximating a value of −54Vdc is developed.

Basic components of the ABC System according to the present inventionare contained within the battery string module 15. An importantcomponent of the battery string module 15 is the high isolationimpedance diodes 17 and 18. Diodes 17 and 18 are connected in serieswith the battery strings 12 and 13 (as used herein, the terms “battery”and “battery string” are used interchangeably to mean any series ofbattery cells, regardless of their particular construction). Thefunction of the isolation impedance means (here shown as diodes 17 and18) is to electrically isolate the battery strings 12 and 13 from thesystem load bus 11. As will become evident from the ensuing description,having the battery strings 12 and 13 isolated from the system load bus11 provides a variety of benefits not available to back-up batterymanagement or monitor systems in which the battery strings are placeddirectly across the load. As explained above, placement of the batterystrings across the load and in parallel, sets up conditions for thermalrunaway, accelerated positive grid corrosion and electrolyte dry-out inVRLA batteries, It also establishes conditions for performance andreliability degradation in batteries of other types.

As will be explained in the following text, the present ABC Systeminvention makes possible: The controlled charging of the battery stringsat a rate that is constant at a given time and for a given condition ofthe battery cells/mono-blocks 16; The avoidance of the potential forthermal runaway; and Negation of performance and reliability degradingproblems that plague prior monitor systems.

Further, these benefits are achieved without depriving the individualbattery strings 12 and 13 from supplying emergency DC powerinstantaneously to the System load 10. Emergency power is supplied inthe event of an AC power outage or any other consequential drop in theprimary DC source 8 output voltage level.

Any of a number of circuit structures and components may be employed toimplement the high isolation impedance means needed in order to removethe batteries 12 and 13 from the influence of the system load busvoltage 11. In the preferred illustrated embodiment, the diode andshunting relay sets Q1, K1 and Q2, K2 are employed to isolate thebatteries 12 and 13 from the load bus voltage 11.

The shunting relay sets K1 and K2 are the normally closed relays 4 and6. The relays 4 and 6 actuate a set of switch contacts 5 and 7respectively when the relays are energized. Further, these relay setsare connected in parallel with the Diodes 17 and 18 and wired to operateas follows: When energized, the switch contacts 5 and 7 have theirindividual contacts 1 and 3 shorted by the switch contacts' wiper. Thiscondition places the diodes 17 and 18 in series with and between thenegative output wiring of the battery strings 12 and 13 and the wiringto the load bus 11; The switch contact 5 has its contact no. 2 wired vialead 39 to the cathode of the diode 18. Therefore, when the relay 4 isde-energized, the battery string 13 is placed directly unto the load bus11; The switch contact 7 has its contact no. 2 wired via lead 40 to thecathode of the Diode 17. Therefore, when the relay 6 is de-energized,the battery string 12 is placed directly unto the load bus 11.

The shunting sets K1 and K2 have break-type normally closed contacts.This feature allows connecting the battery strings 12 and 13 to thesystem load bus 11 in the event that the contactor sets K1 and K2 ortheir control fails (FIG. 2 shows the relays 4 and 6 in the energizedcondition).

In accordance with an important aspect of the present invention is abuck/boost power converter 50, receiving its input power 51 from theload bus 11. It supplies negative DC power 52 to the series regulators23 and 25 that are in series with the battery strings 12 and 13. Itprovides a constant maintenance or “float” current to float charge thebattery strings 12 and 13. The converter 50 may be of conventionalconstruction and with the addition of series regulators 23 and 25, makespossible the application of a constant voltage across the batterycells/mono-blocks. The series regulators 23 and 25 control the voltagelevel and current to the batteries 12 and 13. The float current levelsare dependent upon the type, size and age of the battery. The batterystrings 12 and 13 are connected to the system load bus 11 through thediodes 17 and 18 for all conditions when the primary DC source 8 isunable to supply the system load current. It should be noted that floatcharging at a constant voltage level with current limits prevents damageto the battery cells through thermal runaway or meltdown, as wasexplained above.

In accordance with another feature, a controller 3 performs monitoringand control functions in the ABC system of the present invention. Thecontroller 3 monitors via leads 29 and 30, the voltage of each batterystring 12 and 13. Battery string current through the shunts 32 and 33 ismeasured via leads 35 and 36 from the controller 3. The system load 10current through shunt 48 is measured via lead 49 from the controller 3.The relay sets 4 and 6 are energized or de-energized via the signalleads 45 and 46 respectively from the controller 3. Also, the outputvoltage of the buck/boost converter 50 can be adjusted via the signallead 22 from the controller 3. The series regulators 23 and 25 can beadjusted via control leads 24 and 26 respectively from the controller 3.And last but most importantly, the load bus 11 voltage level ismonitored constantly by dual monitor circuits located on the controlcard circuit board 3 via the monitor lead 44.

The operation of the system according to the present ABC Systeminvention will now be described.

The ABC System of the present invention provides a means to control thefactors that influence the life of a VRLA battery or batteries of othertypes. Controlling and managing the way a VRLA battery is charged, withparticular attention to the float, boost and temperature compensationmethods of charging VRLA batteries, will help maximize their usefullife. Further, the present ABC system provides the means for testing astring or strings of VRLA batteries while monitoring cell voltages. Thisobtains accurate information regarding both a battery's health and stateof readiness (capacity). Also, the ABC system is designed so that anyfailure mode affecting the ability of the system load bus 11 to sustainthe system load 10 will immediately return the power system to astandard rectifier/battery/load parallel configuration. During normaloperation, the primary DC source 8 supplies all load current via thesystem load bus 11 to the system load 10. The VRLA battery strings 12and 13 are connected to the system load bus 11 but displaced/blocked bythe high impedance means of the diodes 17 and 18. The diodes 17 and 18provide an approximate 0.7-0.8 forward voltage drop. This effectivelyremoves the battery strings 12 and 13 from the influence of the PrimaryDC source 8 voltage.

During float charge operation (the predominate mode of operation forcommunications batteries), the buck/boost power converter 50 provides apredetermined substantially constant voltage via 52 to the seriesregulators 23 and 25. The current necessary to float charge the batterystrings 12 and 13 is regulated via feedback circuitry using controlleads 24 and 26 to the series regulators 23 and 25. The current levelsupplied is dependent upon the type, size and ages of the VRLAbatteries. Also and as noted, the float charging current issubstantially constant at a given time and for a given condition of thebattery cells/mono-blocks. The current levels may differ for differenttimes, temperatures and battery conditions.

While maintaining the battery strings 12 and 13 on float charge, acommand signal from the ABC Systems' controller 3 can be sent toincrease the float current output to a higher boost amount. Like thefloat charging current, the boost charging current is constant at agiven time and for a given condition of the battery cells/mono-blocks.However, it can differ for different times, temperatures and batteryconditions. It will also differ depending on the size and ages of theVRLA batteries. In practice, the substantially constant charging current(float and boost) delivered to the battery strings 12 and 13 isdetermined by testing. At the direction and under the control of thecontroller 3, the battery cells/mono-blocks 16 can be individuallytested. The controller 3 determines for a given application of chargingcurrent, or a given period of time, an optimum level of current whichwill cause the battery cells/mono-blocks 16 to deliver optimumperformance over a maximum predicted life. The testing of the batterycells/mono-blocks 16 may include impressing across the individualcells/mono-blocks a constant voltage that would be equal to the primarysource 8 voltage. To accomplish this, the battery string 12 or 13 is putdirectly unto the load bus 11. The cells/mono-blocks 16 are allowed tosettle at the imposed voltage level and are then measured via monitorleads 29 or 30 by the controller 3 to determine if an out of tolerancecondition exists. This test is performed periodically at the discretionof the maintenance manager. This test is most appropriately performedjust prior to the battery strings 12 and 13 scheduled boost charge. Ifno serious out-of-tolerance conditions are exhibited, the boost chargeis only applied to one individual battery string 12 or 13 at a time.

The boost charge is applied to battery string 12 as follows: Thecontroller 3 sends a signal via signal lead 26 to the series regulator25 to decrease its output current to zero. Next, the battery string 13is placed directly on the load bus 11 via a command signal 45 from thecontroller 3 to de-energize the K1 relay 4. This action by thecontroller 3 places the battery string 13 directly on the load bus 11and removes the battery string 12 from the load bus 11 entirely. Now thecontroller 3 signals the buck/boost converter 50 via signal lead 22 toincrease the output voltage level 52 by an amount equal to thepredetermined boost voltage limit increase desired for battery string12. The controller signals the series regulator 23 via signal lead 24 toincrease the regulated current output of the series regulator 23 to 2amps. The voltage level of battery string 12 now begins to increase inresponse to the increased amperage flow through the cells of batterystring 12. The series regulator 23 has been voltage limited by a signalfrom the Controller 3 via signal lead 24. Hence, the voltage level ofbattery string 12 can only increase to a predetermined value. The boostcurrent of 2 amps originally impressed on the battery string 12 by theseries regulator 23, now decreases to a level needed in order tomaintain the battery string 12 at the predetermined boost voltage level.After one and one half-hours have elapsed, the controller 3 sends avoltage adjustment signal via signal lead 24 to the series regulator 23.This signal decreases the voltage limit output of the series regulator23 from a predetermined boost voltage limit to a predetermined floatvoltage limit. The controller 3 also signals the buck/boost converter 50via signal lead 22 to decrease the output voltage level 52 by an amountequal to the predetermined boost voltage limit increase used for batterystring 12. Next, the battery string 13 is removed from the influence ofthe load bus 11 voltage via a command signal 45 from the controller 3 toenergize the K1 relay 4. This action by the controller 3 places both thebattery strings 12 and 13 into a blocked condition in series with thediodes 17 and 18. This condition allows the battery strings 12 and 13access to the load bus 11 but removes them from the influence of theload bus 11 voltage. The controller 3 sends a signal via signal lead 26to the series regulator 25 to increase its output current and continuethe predetermined float output voltage limit.

The frequency of occurrence of a command signal from the Controller 3that initiates the boost charge is determined in either of two ways: Bybattery measurements taken periodically (every 24 hours for example)while on float charge. During this period, VRLA battery cell/mono-block16 voltages are monitored by the ABC System controller 3 to insure thatthe cells/mono-blocks 16 are within a pre-specified voltage tolerancerange. If a group of either 6 cells or a mono block exceeds that range,a boost charge of the associated battery string 12 or 13 is commenced.Note: In this instance, the ABC System can only boost charge one fullbattery string 12 or 13 at a time. Entire battery string 12 and 13 boostcharging (as described previously), takes place at regularly scheduledintervals such as every one or two months.

In the second instance, to insure that the cells/mono-blocks 16 areproperly accepting the higher charge current, the controller 3 monitorsthe cells/mono-blocks 16 voltages as they are being boost charged. Thealternative boost charge, which occurs as needed (as determined byperiodic battery parameter measurements), insures that both the positiveand negative plates of the battery cells/mono-blocks 16 are polarized.Boost charging insures that the cells/mono-blocks 16 are fully chargedand hence, in a fully recombinant state. This minimizes cell “dry out”(water loss), and adds to the useful life of the VRLA battery cells.

When the ABC System is operating in a normal Float charge mode, and theprimary DC power source 8 is unable to supply the total load current,the system load bus 11 voltage begins to decay. A dual voltage monitordetector circuit (here for convenience is considered part of thecontroller 3), detects any voltage drop of the system load bus 11. Thebattery strings 12 and 13 are instantaneously connected to the systemload bus 11 via the diodes 17 and 18 that begin to conduct current fromthe batteries 16 when the load bus 11 voltage decay exceeds 0.7 to0.8Vdc.

The closure of the contacts 1 and 3 of the K1 and K2 shunt relay sets 4and 6 takes place less than 1 second after the initiation of the decayof the system load bus 11 voltage. The transition of battery power tothe system load bus 11 is smooth and without interruption. Throughoutall battery discharges, including emergency discharges, the ABC Systemcontroller 3 is constantly monitoring the battery strings 12 and 13voltage to determine the remaining system reserve capacity. Individualcell/mono-block 16 voltages are also monitored and the data is storedfor later use to determine the health and reserve capacity of theindividual cell/mono-blocks 16.

When the primary DC source 8 returns to operation, it again supplies allload current via the system load bus 11 to the system load 10. The K1and K2 shunt relay sets 4 and 6 remain released (closed). This allowsthe primary DC source 8 constant voltage output to charge the batterystrings 12 and 13. The K1 and K2 shunt relay sets 4 and 6 are notenergized (opened) until the battery string charge current, as detectedby the controller 3 falls below a predetermined level. At that time, theK1 or K2 shunt relay set 4 or 6 operates (opens) and its associatedbattery string 12 or 13 receives the remaining charge via the boostcharge mode as previously described. Once properly charged, the batterystrings 12 and 13 return to the float charge mode.

The actions of the controller 3 outlined above will now be described inmore detail. Individually and for each item monitored, the controlaction of the controller 3 will be described.

System load bus 11 voltage: The controller 3 continuously monitors thesystem load bus 11 voltage via the monitor lead 44 and uses the dataderived as follows: During emergency discharges the controller 3calculates remaining back-up battery capacity. Dual monitor circuitsdetect a voltage decrease and the controller 3 de-energizes the shuntingrelay sets 4 and 6, placing both battery strings 12 and 13 directly ontothe load bus 11.

Battery strings 12 and 13 voltages: The controller 3 monitors thebattery strings 12 and 13 voltages during an emergency discharge, usingthe data derived together with the batteries discharge currentmeasurements to calculate the battery strings' predicted capacity.

Battery cell/mono-block 16 voltages: The controller 3 monitors batterycell/mono-block 16 voltages continuously and uses the data as follows:To detect a battery cell/mono-block 16 voltage that is out of tolerance.During an emergency discharge, the controller 3 stores the data forfuture retrieval to determine the battery cell/mono-block's 16 state ofhealth and capacity.

Battery strings 12 and 13 current measurements: The controller 3measures the battery strings 12 and 13 current levels via monitor leads35 and 36 to current shunts 32 and 33 respectively during batterydischarge and recharge. The controller 3 uses this information todetermine: If the battery strings 12 and 13 are sharing the system load10 properly during a discharge. When the shunt relay sets 4 and 6 shouldbe energized (opened) following a recharge. When a predetermined currentlevel is satisfied, the buck converter 50 is then used to finishcharging the battery strings 12 and 13.

System load current: The system load current level is continuouslymeasured via the monitor lead 49 from the controller 3 to the systemload shunt 48. This data is used during an emergency discharge asfollows: Together with the load bus 11 voltage, it is used to calculatethe remaining back-up battery capacity. To determine that the batterystrings' 12 and 13 current levels are sharing the system load 10properly.

Temperature in the vicinity of battery strings 12 and 13: Ambienttemperature in the near vicinity of the battery strings 12 and 13 iscontinuously measured via monitor leads 35 and 36. The controller 3 usesthe temperature measurements as follows: As an aid to verify that a risein the battery strings' 12 and 13 current is due to temperatureincreases only. Help calculate what temperature compensation changesmight be needed to adjust the battery strings' 12 and 13 float currentlimit value (i.e., to increase or decrease the float current limitvalue).

Shunt relay sets 4 and 6 (K1 and K2): The controller 3 energizes orde-energizes the relays 4 and 6 under the following circumstances: Ifthe controller's 3 dual circuit voltage monitors detect a decay in theload bus 11 voltage exceeding 0.7Vdc, both relays 4 and 6 are sent acommand via leads 45 and 46 from the controller 3 to de-energize. Duringrecharge of the battery strings 12 and 13, the controller 3 detects thata predetermined current level of charge to either battery string 12 or13 has been satisfied. In that instance, the controller 3 sends theappropriate relay 4 or 6 a signal via lead 45 or 46 to energize. When aboost charge of either or both battery strings 12 or 13 is to beinitiated, the controller 3 sends a signal via lead 45 or 46 to theappropriate relay 4 or 6 to energize. Note: During boost charge, onlyone relay 4 or 6 is de-energized at a time. This is because the ABCsystem dictates that only one battery string 12 or 13 can be off-line atany given time. In case of an emergency AC outage, both relays 4 and 6are de-energized and the boost charge is suspended.

Buck converter 50: The controller 3 sends signals via lead 22 to thebuck converter 50 to initiate the following actions: The controller 3chooses one out of the three output voltages the buck converter 50 cangenerate. The controller 3 can signal the buck converter 50 to turn ONor OFF. Series regulators 23 and 25: The controller 3 can adjust thevoltage and current limit settings of the series regulators 23 and 25via control leads 24 and 26 respectively. Note: Voltage and currentlimits placed on the output of the Series Regulators 23 and 25 preventthe battery strings' 12 and 13 cells/mono-blocks 16 from obtainingexcessive amounts of current during charge periods.

The above-described embodiment is merely illustrative of the manypossible specific embodiments that represent applications of the presentinvention. Numerous and varied other arrangements can readily be devisedfollowing the principles of the invention without departing from thespirit and scope of the invention. For example, whereas the controller 3has been described as being remote from the ABC system, to protect thepower supply system from a failure occurring in the controller 3, anumber of the monitoring and control functions e.g., control of theshunt relay sets K1 and K2 and the buck converter 50 to name just twocontrol functions, may be incorporated in a local controller (preferablylocated in the battery string module 15) forming part of the ABC System.

Whereas the invention has been described in a VRLA battery application,the principles of the invention may be employed with flooded lead acidbatteries and rechargeable batteries of other types.

What is claimed is:
 1. A battery management system for a batteryconnected to a load bus wherein the load bus is further connected to aprimary DC source, comprising: variable isolation impedance means havinghigh reverse impedance and variable forward impedance connected incircuit with said battery for electrically isolating said battery fromsaid system load bus without loss of battery back-up capability; controlmeans coupled to the variable isolation impedance so means forcontrolling the forward impedance of the variable isolation impedancemeans in dependence upon a sensed condition; and shunt and connectingmeans responsive to said control means for selectively connecting saidshunt across said variable isolation impedance means to selectivelyconnect said battery to said load bus.
 2. The system defined by claim 1wherein said variable impedance means comprises a gated siliconcontrolled device controlled by said control means.
 3. The systemdefined by claim 2 wherein during recharges of said battery after anemergency discharge into said load, said control means maintains saidshunt across said variable impedance means until the charging currentfalls to a pre-determined charging current level, and thereafter adjuststhe connecting means to disconnect the shunt to electrically isolatesaid battery from said primary DC supply voltage.
 4. A batterymanagement system for use with a primary DC source that supplies aprimary DC supply voltage to a system load bus, said primary DC sourcehaving a parallel back-up battery comprising: control means for sensingbattery and system conditions and for supplying control signals tocomponents of the battery management system; an isolation impedancecircuit connected with said battery and responsive to said control meansfor selectively isolating said battery from said system load bus withoutloss of battery back-up capability; wherein said isolation impedancecircuit comprises an SCR connected in parallel with a controlled switch,said controlled switch having a first state wherein said controlledswitch shunts said SCR to connect said battery to said system load bus;and battery charging means for supplying a charging current to saidbattery while said battery is isolated from said system load bus by saidisolation impedance circuit.
 5. The system defined by claim 4 whereinthe charging current is substantially constant at a given time and for agiven condition of the battery.
 6. The system defined by claim 5 whereinsaid battery charging means responds to commands from said control meansto develop a substantially constant float current and a substantiallyconstant boost current, said float current being lower than said boostcurrent.
 7. The system defined by claim 5 wherein said SCR is controlledby said control means.
 8. The system defined by claim 6 wherein said SCRhas variable forward impedance and wherein said control means adjuststhe forward impedance of said SCR to a first level when said batterycharging means is supplying said float current, and to a second levelwhen said battery charging means is supplying said boost current, saidsecond level being higher than said first Level.
 9. The system definedby claim 4 wherein said battery is a valve-regulated lead acid battery.10. A battery management system for use with a primary DC source whichsupplies a primary DC supply voltage to a system load bus for use by aload, and which primary DC source has a parallel back-up battery havinga battery output voltage, the back-up battery comprising: control meansfor sensing battery and system conditions and for supplying controlsignals to components of the battery management system; isolationimpedance means connected in series with said battery and responsive tosaid control means for selectively isolating said battery from saidsystem load bus without loss of battery back-up capability; and testdischarge means adapted to be connected in series with said battery andsaid system load bus in response to control signals from said controlmeans for developing a predetermined test voltage; wherein the sum ofsaid predetermined test voltage and the battery output voltage beingeffective to discharge a test current into the load, whereby saidbattery may be discharge tested while in a ready state in said system.11. The system defined by claim 10 wherein said isolation impedancemeans includes a gated silicon controlled device controlled by saidcontrol means.
 12. The system defined by claim 11 wherein said batteryis a valve-regulated lead acid battery.
 13. A method of recharging aparallel battery in a back-up battery system after a drop in the primarysupply voltage has caused the battery to be discharged into the load andafter the primary supply voltage has been restored across the load andbattery, the method comprising the steps of; monitoring the flow ofbattery charging current being delivered into the battery from theprimary supply voltage restored across the battery responsive to theflow of battery charging current; applying the primary supply voltageacross the battery until the level of battery current charging falls toa predetermined first charging current level responsive to attainingsaid predetermined first battery charging current level; electricallyisolating the battery from the primary supply voltage without loss ofbattery back-up capability; delivering into the battery a current at apredetermined second current level lower than said first batterycharging current level until the voltage across the battery rises to apredetermined first battery voltage level and responsive to theattainment of said predetermined first battery voltage level; anddelivering into the battery a current at a predetermined third currentlevel lower than said second current level.
 14. The method defined byclaim 13 wherein the charging current is substantially constant at agiven time and for a given condition of the battery.
 15. The methoddefined by claim 14 wherein said isolating of the battery from theprimary supply voltage is achieved by providing a high impedance inseries with said battery.
 16. The method defined by claim 15 whereinsaid high impedance is a gated silicon device, and wherein a gate inputto the gated silicon device is responsive to the primary supply voltageapplied across the load.
 17. The system defined by claim 16 wherein saidbattery is a valve-regulated lead acid battery.
 18. A battery managementsystem for a battery connected to a load bus wherein the load bus isfurther connected to a primary DC source, comprising: a variableisolation impedance circuit connected in circuit with said battery forselectively isolating said battery from said system load bus withoutloss of battery back-up capability, wherein said isolation impedancecircuit comprises an SCR connected in parallel with a controlled switch,said SCR having a high reverse impedance and a variable forwardimpedance said controlled switch having a first state in which saidcontrolled switch shunts said SCR to connect said battery to said systemload bus; and control means coupled to the variable isolation impedancefor controlling the forward impedance of the SCR and the state of thecontrolled switch in dependence upon a sensed battery condition orparameter.
 19. The system defined by claim 3 wherein when the chargingcurrent falls to the predetermined charging current level, the controlmeans adjusts said variable forward impedance to a higher level.
 20. Acharging and monitoring apparatus for use with a battery employed as aback-up power supply to a primary source of power the primary powersource including a load bus for delivering power the apparatuscomprising: a battery module in circuit with the battery for selectivelysupplying charging current thereto, the battery module being in circuitwith the load bus and including an isolating impedance circuit having avariable impedance for selectively isolating the battery from the loadbus of the primary power source; a discharge module for testing thehealth of the battery by selectively discharging the battery to the loadbus; and a controller in circuit with the battery module and thedischarge module for controlling the operation thereof.
 21. An apparatusas defined in claim 20 wherein the battery module includes a powerconverter for selectively supplying the charging current to the batterythe power converter being responsive to the controller to adjust thelevel of the charging current supplied to the battery.
 22. An apparatusas defined in claim 20 wherein the isolation impedance circuit comprisesan SCR and a controlled switch, the controlled switch being connected inparallel with the SCR to selectively form a shunt across the SCRconnecting the battery to the load bus.
 23. An apparatus as defined inclaim 22 wherein the SCR has a high reverse impedance, a first forwardimpedance, and a second forward impedance wherein the second forwardimpedance is higher than the first forward impedance.
 24. An apparatusas defined in claim 23 wherein the controller causes the controlledswitch to form the shunt across the SCR in the event of a failure of theprimary power source.
 25. An apparatus as defined in claim 24 whereinthe controller switches the SCR to the second forward impedance in theevent of a failure of the primary power source.
 26. An apparatus asdefined in claim 22 wherein during recharge after the battery has beenat least partially discharged, the controller causes the controlledswitch to maintain the shunt across the SCR until the charging currentfalls to a predetermined charging level, and thereafter causes thecontrolled switch to change states to again electrically isolate thebattery from the load bus.
 27. An apparatus as defined in claim 20wherein the controlled switch shunts the SCR and connects the battery tothe load bus when a voltage of the load bus drops below a predeterminedlevel.
 28. An apparatus as defined in claim 20 wherein the dischargemodule comprises a power convener for developing a discharge voltagewhich, when summed with a voltage developed by the battery, dischargesthe battery into the load.
 29. An apparatus as defined in claim 20further comprising a controlled switch for selectively disconnecting thedischarge module from the battery.